The present invention relates to satellite communication system downlink waveform structures. In particular, the present invention relates to an efficient satellite downlink waveform including multiple payloads per downlink frame which may be implemented using a hopping beam.
Satellites produce spot beam downlinks that communicate information in time division multiplexed (TDM) frames. In general, the frames include an overhead section and a payload section. The overhead section includes, for example, a guard band and synchronization bits while the payload section carries the “billable” or “useful” data bits. Every time the TDM downlink delivers a payload section in a new frame, the overhead section is retransmitted. Thus, reducing the ratio of overhead to billable data provides an opportunity for increasing the net revenue.
In general, these beams may be hopped in time such that any particular downlink beam may illuminate different geographical spots on the ground, called cells, at any particular time. Such hopping beams permit a single beam to provide broader geographical coverage with a single satellite transmitter chain, saving spacecraft size and weight.
For hopping beams, each beam hop must start with a retransmission of the synchronization sequence. However, any transmission of overhead information necessarily reduces useful data throughput. Such repetition is particularly undesirable in satellite communications, where bandwidth is extremely valuable and useful data throughput is critical to profitability.
In addition to reducing the useful information throughput, the duration of the overhead information represents a hand limit on the minimum delay between delivery of payload sections. In other words, delivery of data that must be split across payload sections in multiple frames incurs an additional delivery delay for every frame. Thus, large messages or data transfers invariably incur significant delivery delays according to the number of frames over which the message or data is distributed.
An additional issue arises for hopping beams in which the hopping pattern is influenced by the traffic demands. Since the hopping pattern may not be fixed, the ground terminal would need some knowledge of the hopping sequence in order to known which transmissions contain payloads destined for that terminal. One such method would be for a network controller to broadcast hop sequences to all terminals, but this entails significant overhead and control. Another approach would be to require each terminal to estimate received downlink power and process those TDMA hops for which the measured power exceeds some threshold. This method has the disadvantage in that terminals at ground cell boundaries may experience very small differences in received power between hops directed to it and hops directed to the adjacent ground cells.
A need exists in the industry for a downlink frame format that addresses the problems noted above and others previously experienced.